1. Field of the Invention
This invention relates to the structure of magnetic recording media. More specifically, the invention relates to the structure of perpendicular recording media (PMR).
2. Description of the Related Art
Magnetic media are widely used in various applications, particularly in the computer and data storage industries, in devices such as hard disk drives and other recording devices. Efforts are continually being made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. In order to produce storage densities in excess of 200 Gb/in2, new recording media structures will be required. In this regard, perpendicular recording media structures (PMR) have been found to be superior to the more conventional longitudinal media in achieving very high bit densities. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium.
US Patent Application Publication US 2002/0058160 discloses a perpendicular magnetic recording medium comprising a combination of an under layer of a laminate structure including at least two layers and a Co-based magnetic layer. The particular combination is selected from the group consisting of i) Fe-containing layer/Ru/magnetic layer, ii) Co-containing layer/Ru/magnetic layer, iii) Ru/Co-containing layer/magnetic layer, iv) Ti-containing layer/Ru/magnetic layer, and v) soft magnetic layer/V or Cr/magnetic layer. A multi-layered structure of magnetic layer/Ru/magnetic layer is used as the magnetic layer included in combinations i) to v) given above. The perpendicular magnetic recording medium of the particular construction permits improving the perpendicular orientation of the Co-based magnetic layer and exhibits a high coercive force and a high reproducing output.
US Patent Application Publication US 2003/0203189 discloses an improved perpendicular magnetic recording medium suitable for high density magnetic recording. In a perpendicular magnetic recording medium comprising a perpendicular magnetic layer and protective layer provided on a non-magnetic substrate via a soft magnetic backlayer, a polycrystalline MgO film is inserted between the soft magnetic backlayer and perpendicular magnetic layer.
US Patent Application Publication US 2004/0000374 discloses a perpendicular magnetic recording medium having a magnetic recording layer with ferromagnetic crystalline grains and nonmagnetic and nonmetallic grain boundary region surrounding the grains. The surface of its under layer, before forming the magnetic recording layer, is exposed to an O2 or N2 atmosphere or an atmosphere of rare gas and O2 or N2, to attach the O2 or N2 as nucleation sites for promoting growth of the nonmagnetic and nonmetallic region. By forming the magnetic recording layer thereafter, both ferromagnetic crystalline grains and the nonmagnetic and nonmetallic grain boundary region are formed from the initial stage of the growth of the magnetic recording layer. Thus, a magnetic recording layer having excellent segregation structure can be formed.
US Patent Application Publication US 2004/0001975 discloses a double layered perpendicular recording media having, between a soft magnetic layer and perpendicular magnetic recording layer, an alignment control layer containing an amorphous portion, a crystal size control layer, and an under layer having one of a hexagonal closest packed structure and a face-centered cubic structure.
US Patent Application Publication US 2004/0072031 discloses a magnetic recording medium including a magnetic recording layer and a substrate that supports the magnetic recording layer. At least two under layers including a nonmetallic under layer are placed between the magnetic recording layer and the substrate. The perpendicular magnetic recording medium uses a double-layered or tri-layered under layer. Accordingly, a perpendicular magnetic recording layer can have a high perpendicular magnetic anisotropic energy constant Ku due to a third under layer and have small crystal grains and a small exchange coupling due to a second under layer below the third underlayer. Thus, the perpendicular magnetic recording layer can have a good thermal stability, high-density recording characteristics, and excellent SNR characteristics.
US Patent Application Publication US 2004/0247945 discloses a perpendicular magnetic recording medium, comprising: (a) a non-magnetic substrate having a surface; and (b) a layer stack formed over the substrate surface, comprising in overlying sequence from the substrate surface: (i) a magnetically soft under layer; (ii) an interlayer structure for crystallographically orienting a layer of a perpendicular magnetic recording material formed thereon; and (iii) at least one crystallographically oriented magnetically hard perpendicular recording layer; wherein the magnetically soft under layer is sputter-deposited at a sufficiently large target-to-substrate spacing and at a sufficiently low gas pressure selected to provide the under layer with a smooth surface having a low average surface roughness Ra below about 0.3 nm, as measured by Atomic Force Microscopy (AFM).
U.S. Pat. No. 6,858,320 discloses performance of a perpendicular magnetic recording medium, such as an increase in output or a decrease in noise, improved by providing a good orientation of a magnetic recording layer in the perpendicular magnetic recording medium and by reducing an amount of an initial growth layer in the magnetic recording layer. The perpendicular magnetic recording medium includes an under layer, a magnetic recording layer, a protective film, and a liquid lubrication layer, which are sequentially provided on a non-magnetic substrate. The under layer contains non-magnetic NiFeCr or a permalloy-based soft magnetic material.
U.S. Pat. No. 6,699,600 discloses a magnetic recording medium comprising, on a non-magnetic substrate, at least a soft magnetic undercoat film comprising a soft magnetic material; an orientation control film for controlling an orientation of a film directly above; a perpendicular magnetic film in which an axis of easy magnetization is oriented mainly perpendicularly with respect to the substrate; and, a protection film, wherein the perpendicular magnetic film has a structure in which a large number of magnetic grains are separated by a grain boundary layer, and an average separating distance between the magnetic grains along a straight line which connects centers of gravity of mutually neighboring magnetic grains of 1 nm or greater.
U.S. Pat. No. 6,670,056 discloses a perpendicular magnetic recording medium having magnetic characteristics by which an anisotropic magnetic field Hk and a saturation magnetization Ms satisfy the requirement 2<Hk/4.pi.Ms<5, letting .alpha. be the inclination of an MH loop when a magnetic field is applied perpendicularly, the anisotropic magnetic field Hk, the saturation magnetization Ms, and a coercive force Hc satisfy the requirement 0.01<{(.alpha.−1)Hc+4.pi.Ms}/Hk<0.2, and a longitudinal residual magnetization Mr is less than 0.2 times the saturation magnetization Ms.
An article entitled “Very High Density and Low Cost Perpendicular Magnetic Recording Media Including New Layer Structure ‘U-Mag’”, by Matsunuma et al., IEEE Trans on Magnetics, Vol 41, No. 2, February 2005, discloses a new layered structure, named “U-Mag”, for perpendicular recording media. The stacked films include a very thin ferromagnetic Co layer (2 nm) and lattice spacing control layers. The structure formed with a 100 nm soft magnetic under layer with high coercivity shows a higher signal to noise ratio than a medium using a conventional Ru underlayer.
Fabrication of one type of prior art perpendicular recording media (PMR) employs a Ru hcp (hexagonal closed packed) under layer to control the c-axis orientation of the Co based magnetic recording layer. The Ru growth and its structural characteristics are critical for achieving the desired magnetic and microstructural properties of the recording medium. The Ru hcp under layer is grown on a seed layer such as Ni80Fe20 and the Ru growth parameters (the sputter pressure, growth rate, etc) are optimized to improve its crystallographic properties and to improve lattice matching to the Co alloy layer. FIG. 1 (Prior Art) shows a prior art perpendicular media architecture 100 in which dual hcp under layers 114a, 114b of Ru grown at different sputter pressures and having different thicknesses are employed to control the microstructural properties of the PMR CoPtCr—SiO2 magnetic recording layer 118. The use of such Ru hcp dual under layers to improve recording media performance is consistent with the teachings of Hikosaka, U.S. Pat. No. 6,670,056. Typically, layer 114a is 5 nm thick, grown at a sputtering pressure of 5 mTorr, and layer 114b is 12 nm thick, grown at a sputtering pressure of 55 mTorr. Alternatively, layers 114a and 114b may be combined into a single layer, grown at a single sputtering pressure. The structure shown in FIG. 1 includes a 2 nm thick layer 112 of Ni80Fe20 to nucleate the desired growth orientation of the subsequently grown Ru hcp under layers 114. Layer 112 is grown over a pair of 75 nm CoTaZr soft under layers (SUL) 106 and 110, which are separated by a 0.7 nm Ru layer 108. The SUL layers 106 and 110 are deposited over substrate 102 and AlTi layer 104. Overcoat layers 120 are deposited on top of recording layer 118, and include protective and lubricating components. The layers intercalated between the top of the SUL 110 and the CoPtCr—SiO2 alloy magnetic recording layer 118 may be referred to as the Inter-Mediate Layers or IML for short.
Variations of this structure have been implemented to fit media fabrication constraints (such as number of available sputter targets) and include replacing the dual hcp under layers by a single Ru hcp under layer grown at an optimized high sputter pressure. In addition other workers in the field have replaced the Ni20Fe80 nucleation layer by different Ni alloys such as NiCr, NiV, NiW. Furthermore different alloys such as NiAl, CrTa and CuNb have been investigated as Ru nucleation layers in order to improve its micro-structural properties.
Ru is chosen as a component in prior art IMLs for a number of reasons. Firstly, the Ru hcp under layer 114 is produced with a strong crystallographic texture as a result of its basal plane being predominantly aligned parallel to the film plane of the Ni20Fe80 nucleation layer 112. Secondly, Ru is chosen to achieve lattice matching between its hexagonal plane and that of the CoPtCr—SiO2 alloy in magnetic recording layer 118. A representative schematic showing a typical hcp structure and lattice constants a and c are found in FIG. 3 (Prior Art). Thirdly, control of grain size and grain size distribution of the Ru hcp under layer 114 is employed to control the grain size of the CoPtCr—SiO2 alloy in magnetic recording layer 118. Although implementation of Ru in prior art IML's has led to significant improvements in perpendicular media performance, there exists inherent limitations to this solution. The lattice parameters of the Ru hcp under layer 114 can be modified only up to a certain point by altering growth conditions. The evolution of defects, faults and stress relaxation imposes a hard limit to lattice parameter changes. Additionally, control of the grain size, grain size distribution, and nucleation kinetics required for the formation of the desired crystallographic orientation is limited when employing prior art processing technology.
What is needed is an IML structure that provides micro-structural improvements of the magnetic recording layer and a large improvement in magnetic recording performance when compared to prior art PMR media employing the same magnetic recording layer.